The present invention relates to frequency synthesizers, and more particularly to frequency synthesizers which utilize a phase locked loop having a voltage controlled oscillator that must be coarse- or pre-tuned in order to bring the output signal of the oscillator within the capture or lock-up range of the phase locked loop.
Frequency synthesizers using phase locked loops with voltage controlled oscillators are well known. The phase locked loop basically comprises a feedback circuit for producing a fine tune control signal which is applied to the voltage controlled oscillator and operates to bring the output signal of the oscillator into phase lock with a reference signal. Due to the properties of the components which make up the phase locked loop, the loop has an inherent capture range wherein the output signal from the voltage controlled oscillator must be within a predetermined range of frequencies from the reference signal before the loop can operate to bring the oscillator signal into phase lock with the reference signal. When the tuning range of the voltage controlled oscillator is greater than the capture range of the phase locked loop, it is a frequent requirement that the voltage controlled oscillator be coarse- or pre-tuned to bring the frequency of its output signal within the capture range of the phase locked loop so that the system will be able to acquire phase lock. This requirement becomes more stringent as the ratio of the oscillator tuning range to the loop capture range increases.
Previous methods for pretuning the voltage controlled oscillator in a frequency synthesizer have generally involved the generation of a coarse tuning voltage as a fixed function of the desired output frequency. The value of the coarse tuning voltage is predetermined for each desired output frequency. Difficulties have arisen in the use of these prior art methods because misalignment of the oscillator, aging of components, oscillator drift and temperature effects can often result in the need to coarse tune the oscillator with a voltage signal which is different from the predetermined signal for a given output frequency. A predetermined fixed tuning voltage cannot adapt to the changing conditions brought about by these factors, and therefore requires periodic adjustment.
In addition, the characteristics of frequency synthesizers and voltage controlled oscillators vary from one synthesizer to the next. This factor requires that the predetermined tuning voltages be established individually for each synthesizer during production, thus increasing the cost and time of manufacturing. This problem is further compounded when the tuning characteristics of the oscillator are non-linear, unpredictable or inconsistent.
Therefore, it is an object of the present invention to provide a novel method and apparatus for pretuning a voltage controlled oscillator in a phase locked loop frequency synthesizer which overcomes the problems associated with the use of a fixed pretuning voltage generated as a function of the desired output frequency.
It is another object of the present invention to provide a novel method and apparatus for pretuning a voltage controlled oscillator which is automatic and adapts to the changing conditions brought about by misalignment of the oscillator, aging of components, oscillator drift, the effect of temperature on the various circuit components as well as the inherent differences found from synthesizer to synthesizer.
It is a further object of the present invention to provide a novel method and apparatus for automatically pretuning a voltage controlled oscillator in a variety of different types of frequency synthesizers having different tuning characteristics.
Another prior art technique for pretuning a voltage controlled oscillator utilizes a step or staircase voltage signal as the coarse tuning signal. The staircase signal is applied as a control signal to the voltage controlled oscillator until the output signal from the oscillator is within the capture range of the phase locked loop. U.S. Pat. No. 3,825,855, issued to Basset et al, discloses a frequency synthesizer which is exemplary of this prior art.
One problem associated with the use of a staircase coarse tuning signal is the amount of time that is required to pretune the voltage controlled oscillator to the proper value. Since the coarse tuning voltage is varied by only an incremental voltage value during each clock period of the pretuning operation, a substantial amount of time may be required to bring the oscillator output signal to the proper frequency, particularly if the proper pretuning signal is at or near one end of the oscillator tuning range. The time involved becomes an important factor when rapid frequency changes are required, for example, when shifting a transceiver from the transmitting to the receiving mode of operation.
It is therefore another object of the present invention to provide a novel method and apparatus for automatically adaptively pretuning a voltage controlled oscillator which avoids the time constraints involved with the use of a staircase coarse tuning signal.
It is yet a further object of the present invention to provide a novel method and apparatus for pretuning the voltage controlled oscillator in a frequency synthesizer which is capable of changing the output signal of the voltage controlled oscillator from a first frequency to a second frequency in a relatively short period of time.
These and other objects and advantages of the present invention will become apparent to one of ordinary skill in the art to which the present invention pertains upon a perusal of the following description when taken in conjunction with the accompanying drawings.